EP0400992B1

EP0400992B1 - Method for driving display device

Info

Publication number: EP0400992B1
Application number: EP90305884A
Authority: EP
Inventors: Hiroshi Maeda; Takuro Omori
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1989-05-30
Filing date: 1990-05-30
Publication date: 1996-03-20
Anticipated expiration: 2010-05-30
Also published as: CA2017757A1; JP2637822B2; EP0400992A2; DE69025983T2; EP0400992A3; JPH032722A; CA2017757C; US5412395A; DE69025983D1

Description

The present invention relates to a method for driving such display devices as a liquid crystal display device, thin film EL display device, and the like.
For a liquid crystal display device and the like, a frame thinning out system is well-known as one of driving methods for enabling such a display panel to display a gradationally toned image with several brightness levels on its screen.
The frame thinning out system turns on pixels only in a number of frames corresponding to a level of brightness to be displayed out of all frames within an integration time period consisting of a plurality of frame time periods so as to visually obtain a medium level of brightness in each integration time period.
The applicant of the invention has already proposed a driving method to reduce flicker on the screen by grouping all pixels of the display device into groups each of which is composed of a plurality of pixels contiguous to each other and limiting the total number of pixels to be turned on for each frame time period within the same group according to its display data as a display driving method improving the 8 conventional frame thinning out system (see EP-A-0387033).
Table 1 shows gradation rules (rules of turning pixels on or off for which frame time period) in case of making a gradational display by setting eight frame time periods as one integration time period by means of this driving method. In this case, all pixels on the screen are grouped in to groups each of which is composed of four pixels contiguous to each other as shown by arrow II on the display panel composed of a plurality of pixels arranged in the form of matrix on it as shown schematically in the plan view of Figure 1.
In Table 1, numeral 1 indicates turning a pixel on and numeral 0 indicates turning a pixel off.
Figure 2 is a plan view schematically showing a magnification of one group of pixels shown by arrow II in the display panel. In the grouping, when setting the pixel in the upper left hand corner of the screen of the display panel 1 as a pixel in the 0th row and 0th column, the pixel arrangement is set so that in Figure 2 the two upper pixels A and B are in an even-numbered row, the two lower pixels C and D are in an odd-numbered row, the two left pixels A and C are in an even-numbered column, and the two right pixels B and D are in an odd-numbered column.
Figure 3 is a schematic diagram showing a gradation rule of each of pixels A to D within one group in case of displaying the 6/8 gradation level all over the screen of the display panel 1 according to the gradation rule.
As known from Figure 3, in case of displaying the 6/8 gradation level, three pixels B, C and D are turned on (the pixel A is turned off) for the first frame time period, three pixels A, C and D are turned on (the pixel B is off) for the second frame time period, three pixels A, B and D are turned on (the pixel C is off) for the third frame time period, and three pixels A, B and C are turned on for the fourth frame time period. The rule for the first to fourth frame time periods is also applied to the fifth to eighth frame time periods, and after this the cycle is repeated which has the eight frame time periods as an integration time period. Namely, in case of displaying the 6/8 gradation level, three pixels are always turned on for each frame time period and each of pixels A to D are turned on equally six times for eight frame time periods.
In this case brightness level as a whole of a unit image area of one group composed of pixels A to D is as shown in Figure 4.
Namely, since three pixels are turned on throughout all frame time periods, the brightness level is always 3/4. Accordingly, the 6/8 brightness level is displayed on the average during one integration time period consisting of eight frame time periods and brightness level of the group does not vary as a whole of the unit image area, so flicker is reduced.
In the above-mentioned driving method previously proposed, however, as shown in Figure 3, in case of the 6/8 brightness level the number of pixels turned on in a subgroup of pixels A and B in the upper even-numbered row out of the pixels A to D within one group for each frame time period is different from the number of pixels turned on in a subgroup of pixels C and D in the lower odd-numbered row. As a result, there has been a problem that quality of the display is deteriorated depending on a viewing angle in case that the above-mentioned driving method is applied to such a capacitive display device as a liquid crystal display device.
Namely, since such a capacitive display device as a liquid crystal display device shows a phenomenon that brightness of the pixel turned on at that time varies according to the number of pixels turned on out of the pixels arranged in one row (common line) on the screen of the display panel 1, difference between the number of pixels turned on in a subgroup of pixels A and B in the upper even-numbered row out of the pixels A to D within one group and the number of pixels turned on in a subgroup of pixels C and D in the lower odd-numbered row causes such deterioration of quality of the display that brightness of the pixels vary for each frame time period and the bright points seem to be vibrating.
This problem occurs also in case of displaying the other gradation levels.
EP-0193728 discloses a method of driving an array of pixels in which the number of frames during which a pixel is driven corresponds to the gradation level for that pixel. The pixels are divided into two interlaced chequerboard arrays which are driven alternately to reduce flicker. Although the number of pixels driven in each line may be constant the method produces substantial variations in brightness within a group for partial gradation levels.
An object of the present invention is to provide a display driving method which makes it possible to display a high quality image with no noticeable flicker and no delicate shades of brightness among pixels of the same gradation level even in case of such a capacitive display device as a liquid crystal device.
The invention provides a method for driving a display device including an array of display pixels to generate a gradationally toned display of several brightness levels, comprising:

setting a frame time period, during which each of the pixels is controlled to display an image, which is shorter than the minimum period at which flicker is perceived;
setting an integration time period equal to a plurality of frame time periods;
grouping the pixels into a plurality of groups each group consisting of a plurality of sub-groups each sub-group containing more than two pixels selected from a respective one of at least two adjacent lines of pixels; and
driving each pixel for a number of frame time periods corresponding to a brightness level for that pixel in each integration period, wherein the pixels of each sub-group, when driven at the same brightness level, are driven in such a manner that the number of pixels energized in each sub-group does not vary by more than one between frames, and wherein during each frame time period the number of pixels energized in each line of each group is equal.

Preferably for the or each pixel brightness level at which the brightness level of each group of pixels per frame varies from the average brightness level of the group per integration period, the variation between frames is substantially constant in amount and opposite in direction. Preferably the arrangement of driven pixels varies from frame to frame of an integration period when the pixels of a group are driven at a selected partial brightness level. Preferably the difference between closest pixel brightness levels is equal to the minimum said pixel brightness level.
Preferably the number of pixel brightness levels equals the number of pixels in each group. Preferably the number of frames in the integration period equals the number of pixels in each group.
Preferably each line of each group corresponds to a respective scanning line of the display device.
In one embodiment, the display device is further controlled so that in each group of pixels, predetermined ones of said driven pixels are turned off throughout all frame periods, said predetermined pixels forming a checker pattern over the array of display pixels.
Accordingly, in case of viewing a plurality of pixels all over one group as a unit image area, a high quality gradational display without noticeable flicker can be obtained. Also since no delicate shades of brightness occur among pixels to display the same gradation level, the quality of the display is improved.
As mentioned above, selecting the number of pixels to be turned on out of plural pixels contiguous to each other to form one group for each frame time period corresponding to its display data reduces variation of brightness as a whole of one group of plural pixels and makes it possible to obtain a gradationally toned display without noticeable flicker.
Since the number of pixels to be turned on for each frame time period is equal to each other among the subgroups respectively arranged in the scanning lines within the same group, no delicate shades of brightness occur among pixels to display the same gradation level even in case of applying this driving method to such a capacitive display device as a liquid crystal display device; as a result, the quality of the display is much improved.
A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which

Figure 1 is a plan view of the display panel schematically shown to explain grouping of the pixels in a driving method previously proposed as an example,
Figure 2 is a plan view schematically showing a magnification of one group of pixels of the display panel,
Figure 3 is a schematic diagram showing a gradation rule for displaying the 6/8 gradation level in the driving method proposed as an example,
Figure 4 is a schematic diagram showing variation of brightness in one group of pixels controlled on the basis of the gradation rule,
Figure 5 is a block diagram to show roughly configuration of a liquid crystal display device driving method to which a driving method of an embodiment of the invention is applied,
Figure 6 is a plan view schematically showing the display panel of the liquid crystal display device,
Figure 7 is a plan view schematically showing a magnification of one group of pixels of the display panel,
Figure 8 is a schematic diagram showing gradation rules applied to the liquid crystal display device driving method,
Figure 9 is a set of schematic diagrams showing variation of brightness in one group of pixels controlled on the basis of the gradation rules,
Figure 10 is a schematic diagram showing gradation rules in case of display a checker pattern on the basis of the above-mentioned gradation rules,
Figure 11 is a set of schematic diagrams showing variation of brightness in one group of pixels controlled on the basis of the gradation rules,
Figure 12 is a plan view schematically showing a display screen of the checker pattern,
Figure 13 is a schematic diagram showing a gradation rule for the 6/8 gradation level in case of displaying a checker pattern on the basis of the gradation rule in the driving method previously proposed as an example, and
Figure 14 is a schematic diagram showing variation of brightness in one group of pixels controlled on the basis of this gradation rule.

Figure 5 is a block diagram showing roughly a liquid crystal display device driving method to which a driving method of an embodiment of the invention is applied.
In Figure 5, a controller 2, which is a circuit to control driving of a liquid crystal display device 3, consists of a random access memory 4 (hereinafter referred as to RAM) which stores display data including their gradation data, a switch group 5 which sets gradation rules of a frame thinning out system, and a timing signal generating circuit 6 which generates timing signals necessary for making the liquid crystal display device 3 display an image. A host computer 7 is in charge of controlling the timing signal generating circuit 6.
Figure 6 is a plan view schematically showing a display panel 8 of the liquid crystal display device 3 in which a plurality of pixels are arranged in the form of matrix, and Figure 7 is a plan view schematically showing a magnification of one group of pixels shown by arrow VII in the display panel 8.
Figure 8 is a schematic diagram showing gradation rules applied to the liquid crystal display device driving method in case of displaying a uniform gradation throughout the screen (a display in a uniform gradation all over the screen of the display panel 8), and Figure 9 is a set of schematic diagrams showing variation of brightness in a gradational display controlled by the method. Referring to Figure 8 and 9, operation of the liquid crystal display device driving method is described as follows.
Assuming that a gradationally toned display with 9 levels of the 0/8 (0 level), 1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8, and 8/8 (1 level) gradation levels is performed by setting eight frame time periods as one integration time period, a case is described that a unit image area with the 0/8, 1/8, 2/8, 3/8, 4/8, 5/8, 6/8, 7/8, or 8/8 gradation level in brightness is displayed with a group of pixels A, B, C, D, E, F, G and H shown in Figure 7.
When setting the pixel in the upper left hand corner of the screen of the display panel 8 shown in Figure 6 as a pixel in the 0th row and 0th column, all pixels on the display panel 8 are grouped into a plurality of groups each of which is composed of the pixels shown in Figure 7 so that in a group of pixels shown in Figure 7 the upper pixels A, B, G, and H may be in the (2m)th row on the screen, the lower pixels C, D, E, and F may be in the (2m+1)th row on the screen, the pixels A and C in the leftmost column may be in the (4n)th column, the pixels B and D in the 2nd column from the left may be in the (4n+1)th column on the screen, the pixels G and E in the 3rd column from the left may be in the (4n+2)th column on the screen, and the pixels H and F in the rightmost column may be in the (4n+3)th column on the screen (where m and n are respectively integers which are equal to or greater than 0 and independent from each other).
In the switch group 5 shown in Figure 5, the gradation rules shown in Figure 8 are applied to the pixels A to H. Namely, in this case the gradation rules applied to the pixels A to D are exactly the same as the gradation rules applied to the pixels A to D in case of the proposed example shown in Table 1. When setting the driving data of turning on or off the pixels A, B, C, D, E, F, G, and H respectively as a, b, c, d, e, f, g, and h, the gradation rules shown in Figure 8 sets a=e, b=f, c=g, and d=h. Namely, for the pixels in Figure 7, the on/off state of the lower pixels E and F is set to the same as the on/off state of the upper pixels A and B in each frame, and the on/off state of the upper pixels G and H is set to the same as the on/off state of the lower pixels C and D.
In case of displaying the 0/8 gradation level, concretely, the off (no lighting) state is always selected for all of the eight pixels A to H throughout all of the eight frame time periods, and after this the same cycle is repeated.
In case of displaying the 1/8 gradation level, the on (lighting) state is selected for the two pixels A and E in the first frame out of the eight frames, the off state is selected for all of the eight pixels A to H in the second frame, the on state is selected for the two pixels B and F in the third frame, the off state is selected again for all of the eight pixels A to H in the fourth frame, the on state is selected for the two pixels C and G in the 5th frame, the off state is selected again for all of the eight pixels A to H in the 6th frame, the on state is selected for the two pixels D and H in the 7th frame, and the off state is selected for all of the eight pixels A to H in the 8th frame; and after this the same cycle is repeated. Namely, in displaying the 1/8 gradation level, a frame in which two pixels are turned on and another frame in which all of the eight pixels are turned off are alternately displayed. In this case, each of the pixels A to H is turned on equally once (in one frame) during the eight frame time periods.
In case of displaying the 2/8 gradation level, the on state is selected for the pixels A and E in the first frame, for the pixels B and F in the second frame, for the pixels C and G in the third frame, for the pixels D and H in the 4th frame, and the pixel control rules in the first to fourth frames are repeated in the 5th to 8th frames; and after this the same cycle is repeated. Namely, in displaying the 2/8 gradation level, two pixels are turned on in all frames and each of the pixels A to H is turned on equally twice during the eight frame time periods.
In case of displaying the 3/8 gradation level, the on state is selected for the pixels A and E in the first frame, for the pixels B, C, F, and G in the second frame, for the pixels C and G in the third frame, for the pixels A, D, E, and H in the 4th frame, for the pixels D and H in the 5th frame, for the pixels B, C, F, and G in the 6th frame, for the pixels B and F in the 7th frame, and for the pixels A, D, E, and H in the 8th frame; and after this the same cycle is repeated. Namely, in displaying the 3/8 gradation level, a frame in which two pixels are turned on and another frame in which four pixels are turned on are alternately displayed, and each of the pixels A to H is turned on equally three times during the eight frame time periods.
In case of displaying the 4/8 gradation level, the on state is selected for the pixels A, C, E, and G in the first frame and for the pixels B, D, F, and H in the second frame and the gradation rules in the first and second frames are repeatedly applied to the 3rd to 8th frames; and after this the same cycle is repeated. Namely, in displaying the 4/8 gradation level, four pixels are turned on in all of the eight frames and each of the pixels A to H is turned on equally 4 times during the 8 frames.
In case of displaying the 6/8 gradation level, the on state is selected for the pixels B, C, D, F, G, and H in the first frame, for the pixels A, C, D, E, G, and H in the second frame, for the pixels A, B, D, E, F, and H in the third frame, for the pixels A, B, C, E, F, and G in the 4th frame, and the gradation rules in the first to fourth frames are repeatedly applied to the 5th to 8th frames; and after this the same cycle is repeated. Namely, in displaying the 6/8 gradation level, six pixels are turned on in all the frames and each of the pixels A to H is turned on equally 6 times during the eight frame time periods.
In case of displaying the 7/8 gradation level, the on state is selected for the pixels B, C, D, F, G, and H in the first frame, for all of the pixels A to H in the second frame, for the pixels A, C, D, E, G, and H in the third frame, for all of the pixels A to H in the fourth frame, for the pixels A, B, D, E, F, and H in the 5th frame, for all of the pixels A to H in the 6th frame, for the pixels A, B, C, E, F, and G in the 7th frame, and for all of the pixels A to H in the 8th frame; and after this the same cycle is repeated. Namely, in displaying the 7/8 gradation level, a frame in which six pixels are turned on and another frame in which eight pixels are turned on are alternately displayed and each of the pixels A to H is turned on equally 7 times during the eight frame time periods.
In case of displaying the 8/8 gradation level, the on state is always selected for all of the eight pixels A to H during the eight frame time periods, and after this the same cycle is repeated.
According to these gradation rules, the controller 2 drives the eight pixels A to H forming one group in the display panel 8 of the liquid crystal display device 3. In these cases, brightness levels as a whole of one group composed of the 8 pixels A to H in the display panel 8 are as shown in Figures 9 (a), (b), (c), (d), (e), (f), (g), and (h).
That is to say, in case of displaying the 0/8 gradation level, since all of the 8 pixels A to H are turned off throughout all frame time periods, the brightness level is always 0 as shown in Figure 9 (a). Accordingly, the average brightness level is also 0 during one integration time period consisting of 8 frame time periods. In this case, since the brightness level does not vary, no flicker occurs.
In case of displaying the 1/8 gradation level, as shown in Figure 9 (b), since two pixels are turned on in the first frame the brightness level is 2/8 and since all of the 8 pixels are turned off in the second frame the brightness level becomes 0, and this cycle is repeated. As a result, the 1/8 brightness level is displayed on the average during one integration time period consisting of 8 frame time periods. And since a frequency of alternating the 2/8 and 0 brightness levels is a half of the frame frequency and (quadruple) greater than the frequency having 8 frame periods as one integration period, flicker is reduced so much.
In case of displaying the 2/8 gradation level, as shown in Figure 9 (c), since two pixels are always turned on throughout all frame time periods, the brightness level is always 2/8. Accordingly, the average brightness level is also 2/8 during one integration time period consisting of 8 frame time periods. In this case, since the brightness level does not vary, no flicker occurs.
In case of displaying the 3/8 gradation level, as shown in Figure 9 (d), since two pixels are turned on in the first frame the brightness level is 2/8 and since four pixels are turned on in the second frame the brightness level becomes 4/8, and this cycle is repeated. As a result, the 3/8 brightness level is displayed on the average during one integration time period consisting of 8 frame time periods. And since a frequency of alternating the 2/8 and 4/8 brightness levels is a half of the frame frequency and (quadruple) greater than the frequency having 8 frame periods as one integration period, flicker is reduced so much.
In case of displaying the 4/8 gradation level, as shown in Figure 9 (e), since four pixels are always turned on throughout all frame time periods, the brightness level is always 4/8. Accordingly, the average brightness level is also 4/8 during one integration time period consisting of 8 frame time periods. In this case also, since the brightness level does not vary, no flicker occurs.
In case of displaying the 6/8 gradation level also, as shown in Figure 9 (f), since six pixels are always turned on throughout all frame time periods, the brightness level is always 6/8. In this case also, since the brightness level does not vary, no flicker occurs.
In case of displaying the 7/8 gradation level, as shown in Figure 9 (g), since six pixels are turned on in the first frame the brightness level is 6/8 and since all of the 8 pixels are turned on in the second frame the brightness level becomes 8/8, and this cycle is repeated. As a result, the 7/8 brightness level is displayed on the average during one integration time period consisting of 8 frame time periods. And since a frequency of alternating the 6/8 and 8/8 brightness levels is a half of the frame frequency and greater than the frequency having 8 frame periods as one integration time period, flicker is reduced so much.
And in case of displaying the 8/8 gradation level, as shown in Figure 9 (h), since all of the 8 pixels are always turned on throughout all frame time periods, the brightness level is always 8/8. Accordingly the average brightness level is also 8/8 during one integration time period consisting of 8 frame time periods. In this case also, since the brightness level does not vary, no flicker occurs.
As mentioned above, since the on/off state of the pixels A and B in the upper row out of the pixels A to H of one group is the same as the on/off state of the pixels E and F in the lower row and the on/off state of the pixels C and D in the lower row is the same as the on/off state of the pixels G and H in the upper row, the number of pixels turned on out of the pixels A, B, G, and H in the upper row and the number of pixels turned on out of the pixels C, D, E, and F in the lower row within one group are always equal to each other. Accordingly, in case that the same brightness level of display is displayed all over the screen, the number of pixels turned on out of the pixels arranged in one scanning line in a frame is always the same as that in any other frame. As a result, brightness of pixels turned on does not happen to vary in each frame and any delicate shades of brightness do not occur among pixels.
Figure 10 is a schematic diagram showing gradation rules for displaying a checker pattern as shown in Figure 12 on the basis of the gradation rules shown in Figure 8, and Figure 11 is a set of schematic diagrams showing variation of brightness level in a gradational display obtained by the gradation rules in Figure 10.
In Figure 12, a pixel to be turned on out of pixels on the display panel 8 is indicated as a blank box, and a pixel to be turned off is indicated as a hatched box. The pixel in the upper left hand corner of the screen and pixels alternately disposed following it are turned on. Accordingly, the gradation rules shown in Figure 10 are equivalent to those according to which the pixels B, C, E, and H out of the pixels A to H controlled according to the gradation rules shown in Figure 8 are turned off throughout all frame time periods.
As mentioned above, since the gradation rules shown in Figure 8 are set so that the on/off state of the pixels A and B out of one group of pixels shown in Figure 7 may be equal to the on/off state of the pixels E and F and the on/off state of the pixels C and D may be equal to the on/off state of the pixels G and H, the pixels F and G to be set in the same on/off state as the pixels B and C are not changed to be turned off among the pixels E, F, G, and H in order to display a checker pattern, even in case that the pixels B and C to be originally turned on are changed to be turned off in order to display a checker pattern among the pixels A, B, C, and D. Namely, it is the pixels E and H that are changed to be turned off among the pixels E, F, G, and H in order to display a checker pattern in spite of being originally turned on.
On the other hand, even in case that among the pixels E, F, G, and H the pixels E and H are changed to be turned off in order to display a checker pattern in spite of being originally turned on, among the pixels A, B, C, and D the pixels A and D to be set in the same on/off state as the pixels E and H are not changed to be turned off in order to display a checker pattern. Accordingly, the number of pixels turned on in each frame according to the gradation rules in case of displaying a checker pattern is reduced to one half as compared with that according to the gradation rules shown in Figure 8, but the corresponding relation between the number of pixels turned on and the gradation levels is kept equal in both cases.
Namely, in Figure 10, in case of displaying the 0/8 gradation level, the on/off state of the pixels is the same as that in Figure 8, but in case of displaying the 1/8 gradation level, the pixel A is turned on in the 1st frame, all the pixels are turned off in the 2nd frame, the pixel F is turned on in the 3rd frame, all the pixels are turned off in the 4th frame, the pixel G is turned on in the 5th frame, all the pixels are turned off in the 6th frame, the pixel D is turned on in the 7th frame, and all the pixels are turned off in the 8th frame. Namely, in displaying the 1/8 gradation level, a frame in which one pixel is turned on and another frame in which all of the 8 pixels A to H are turned off are alternately displayed.
In case of displaying the 2/8 gradation level, the on state is selected for the pixel A in the 1st frame, for the pixel F in the 2nd frame, for the pixel G in the 3rd frame, for the pixel D in the 4th frame, and the same gradation rules as the 1st to 4th frames are applied to the 5th to 8th frames. Namely, in displaying the 2/8 gradation level, one pixel is turned on in all frames.
In case of displaying the 3/8 gradation level, the on state is selected for the pixel A in the 1st frame, for the pixels F and G in the 2nd frame, for the pixel G in the 3rd frame, for the pixels A and D in the 4th frame, for the pixel D in the 5th frame, for the pixels F and G in the 6th frame, for the pixel F in the 7th frame, and for the pixels A and D in the 8th frame. Namely, in displaying the 3/8 gradation level, a frame in which one pixel is turned on and another frame in which two pixels are turned on are alternately displayed.
In case of displaying the 4/8 gradation level, the on state is selected for the pixels A and G in the 1st frame, for the pixels D and F in the 2nd frame, and after this the same cycle is repeated. Namely, in displaying the 4/8 gradation level, two pixels are turned on in all frames.
In case of displaying the 6/8 gradation level, the on state is selected for the pixels D, F, and G in the 1st frame, for the pixels A, D, and G in the 2nd frame, for the pixels A, D, and F in the 3rd frame, for the pixels A, F, and G in the 4th frame, and the same gradation rules as the 1st to 4th frames are applied to the 5th to 8th frames. Namely, in displaying the 6/8 gradation level, three pixels are turned on in all frames.
In case of displaying the 7/8 gradation level, the on state is selected for the pixels D, F, and G in the 1st frame, for the pixels A, D, F, and G in the 2nd frame, for the pixels A, D, and G in the 3rd frame, for the pixels A, D, F, and G in the 4th frame, for the pixels A, D, and F in the 5th frame, for the pixels A, D, F, and G in the 6th frame, for the pixels A, F, and G in the 7th frame, and for the pixels A, D, F, and G in the 8th frame. Namely, in displaying the 7/8 gradation level, a frame in which three pixels are turned on and another frame in which four pixels are turned on are alternately displayed.
And in case of displaying the 8/8 gradation level, the on state is selected for the pixels A, D, F, and G in all frames. Namely, in displaying the 8/8 gradation level, four pixels are turned on in all frames.
In these cases, brightness levels as a whole of one group composed of the 8 pixels A to H in the display panel 8 are as shown in Figure 11 (a), (b), (d), (c), (e), (f), (g), and (h).
That is to say, in case of displaying the 0/8 gradation level, since all of the pixels A to H are turned off throughout all frame time periods, the brightness level is always 0 as shown in Figure 11 (a).
In case of displaying the 1/8 gradation level, as shown in Figure 11 (b), since one pixel is turned on in the first frame the brightness level is 1/8 and since all of the 8 pixels are turned off in the 2nd frame the brightness level becomes 0, and this cycle is repeated. As a result, the 1/16 brightness level is displayed on the average during one integration time period consisting of 8 frame time periods. And since a frequency of alternating the 1/8 and 0 brightness levels is a half of the frame frequency and (quadruple) greater than the frequency having 8 frame periods as one integration period, flicker is reduced so much.
In case of displaying the 2/8 gradation level, as shown in Figure 11 (c), since one pixel is always turned on throughout all frame time periods, the brightness level is always 1/8. Accordingly the average brightness level is also 1/8 during one integration time period consisting of 8 frame time periods. In this case, since the brightness level does not vary, no flicker occurs.
In case of displaying the 3/8 gradation level, as shown in Figure 11 (d), since one pixel is turned on in the 1st frame the brightness level is 1/8 and since two pixels are turned on in the 2nd frame the brightness level becomes 2/8, and this cycle is repeated. As a result the 3/16 brightness level is displayed on the average during one integration time period consisting of 8 frame time periods. And since a frequency of alternating the 1/8 and 2/8 brightness levels is a half of the frame frequency and greater than the frequency having 8 frame periods as one integration period, flicker is reduced so much.
In case of displaying the 4/8 gradation level, as shown in Figure 11 (e), since two pixels are always turned on throughout all frame time periods, the brightness level is always 2/8. Accordingly the average brightness level is also 2/8 during one integration time period consisting of 8 frame time periods. In this case also, since the brightness level does not vary, no flicker occurs.
In case of displaying the 6/8 gradation level, as shown in Figure 11 (f), since three pixels are always turned on throughout all frame time periods, the brightness level is always 3/8. In this case also, since the brightness level does not vary, no flicker occurs.
In case of displaying the 7/8 gradation level, as shown in Figure 11 (g), since three pixels are turned on in the 1st frame the brightness level is 3/8 and since four pixels are turned on in the 2nd frame the brightness level becomes 4/8, and this cycle is repeated. As a result the 7/16 brightness level is displayed on the average during one integration time period consisting of 8 frame time periods. And since a frequency of alternating the 3/8 and 4/8 brightness levels is a half of the frame frequency and greater than the frequency having 8 frame periods as one integration period, flicker is reduced so much.
In case of displaying the 8/8 gradation level, as shown in Figure 11 (h), since four pixels are always turned on throughout all frame time periods, the brightness level is always 4/8. Accordingly the average brightness level is also 4/8 during one integration time period consisting of 8 frame time periods. In this case also, since the brightness level does not vary, no flicker occurs.
By the way, in case of displaying the checker pattern shown in Figure 12 according to the gradation rules of the previously proposed example shown in Table 1, flicker is noticeable on the screen. This is described below taking the 6/8 gradation level of display as an example.
Figure 3 is a schematic diagram showing the gradation rule for the 6/8 gradation level in Table 1. The gradation rule for displaying a checker pattern as shown in Figure 12 is indicated by a schematic diagram shown in Figure 13.
Namely, the schematic diagram shown in Figure 13 is equalent to the case of turning off the pixels B and C in all frames in the schematic diagram shown in Figure 3. Accordingly, brightness as a whole of the unit image area of one group composed of the four pixels A to D is as shown in Figure 14. In Figure 14, the one pixel D is turned on in the first frame and the brightness level is 1/4, the two pixels A and D are turned on in the second frame and the brightness level is 2/4, the two pixels A and D are turned on in the third frame also and the brightness level is 2/4, the one pixel A is turned on in the 4th frame and the brightness level is 1/4, and the gradation rules in the first to fourth frames are repeatedly applied to the fifth to eighth frames. Namely, in case of displaying a checker pattern according to the gradation rule for the 6/8 gradation level, the brightness level is 2/4 in consecutive two frames and is 1/4 in the following consecutive two frames, and this cycle is repeated. Accordingly a frequency of alternating the 1/4 and 2/4 brightness levels is one fourth of the frame frequency, and is less than the frequency in case of the same 6/8 brightness level in the above-mentioned embodiment. Namely, in case that the frame frequency is 80 Hz or so, the frequency of alternating becomes 20 Hz or so, and as a result flicker comes out to be very noticeable and causes deterioration of quality of the display.
As compared with this, in case of the embodiment, the frequency of alternating variation of brightness level is at least a half or so of the frame frequency, so flicker is not so much noticeable as mentioned above.

Claims

A method for driving a display device including an array of display pixels to generate a gradationally toned display of several brightness levels, comprising:
setting a frame time period, during which each of the pixels is controlled to display an image, which is shorter than the minimum period at which flicker is perceived;

setting an integration time period equal to a plurality of frame time periods;

grouping the pixels into a plurality of groups each group consisting of a plurality of sub-groups each sub-group containing more than two pixels selected from a respective one of at least two adjacent lines of pixels; and

driving each pixel for a number of frame time periods corresponding to a brightness level for that pixel in each integration period, wherein the pixels of each sub-group, when driven at the same brightness level, are driven in such a manner that the number of pixels energized in each sub-group does not vary by more than one between frames, and wherein during each frame time period the number of pixels energized in each line of each group is equal.
A method as claimed in claim 1 wherein for the or each pixel brightness level at which the brightness level of each group of pixels per frame varies from the average brightness level of the group per integration period, the variation between frames is substantially constant in amount, and opposite in direction.
A method as claimed in claim 1 or claim 2 wherein the arrangement of driven pixels varies from frame to frame of an integration period when the pixels of a group are driven at a selected partial brightness level.
A method as claimed in any one of the preceding claims wherein the increment between pixel brightness levels is equal to the minimum said pixel brightness level.
A method as claimed in any one of the preceding claims wherein the number of pixel brightness levels equals the number of pixels in each group.
A method as claimed in any one of the preceding claims wherein the number of frames in the integration period equals the number of pixels in each group.
A method as claimed in any one of the preceding claims wherein each line of each group corresponds to a respective scanning line of the display device.
A method according to any preceding claim wherein the display device is further controlled so that in each group of pixels, predetermined ones of said drive pixels are turned off throughout all frame periods, said predetermined pixels forming a checker pattern over the array of display pixels.